FIG. 1 shows an example of a conventional machining apparatus having a rotating shaft (c-axis) that is controlled by a numerical control apparatus; this apparatus is a kind of a composite lathe that continuously performs not only lathing but also milling the workpiece such as drilling. In FIG. 1, reference numeral 1 denotes an induction motor, 2, a servo motor; 3, a main shaft (c-axis); 5, a workpiece mounted on the main shaft 3; 6, a drill; 7, a device for driving the drill 6 which is detachably mounted thereon; 9, a cutting tool and 10, a turret for controlling the movement in directions of X- and Z-axes, of the drill 6, device 7 and cutting tool 9.
Machining operation of the composite lathe shown in FIG. 1 will be hereinafter described with reference to FIG. 2. illustrating an example of the finished configuration of the workpiece.
In order to permit the composite lathe to work as a lathe, the induction motor 1 is first coupled to the main shaft 3 so as to rotate the workpiece 5 at a rotation speed determined by an applied command. Then, the cutting tool 9 is mounted on the turret 10 and lathes the workpiece 5 to provide portions 5a and 5b as shown in FIG. 2, with its movement in the directions of X- and Z-axes being controlled by a numerical control apparatus NC 21 connected to the lathe. Subsequently, in order to shift the operation mode to a continuous milling operation, the shaft 3 is then coupled to the servo motor 1 and the position of the workpiece 5 is numerically controlled by rotating the shaft (C-axis) serving as a positioning control rotating shaft according to a control command from the numerical control apparatus NC 21. In this case, the drill 6 is mounted on the device 7 and is allowed to rotate at a high speed. Under the control of the numerical control apparatus NC 21, the main shaft 3 is rotated in a predetermined direction by an increment of 45 degrees so that it will be successively positioned for the eight holes 5c1 to 5c8 shown in FIG. 2. After the main shaft 3 has been positioned in such a way as described above, the workpiece 5 is milled to produce the finished shape shown in FIG. 2, with the movement of turrent 10 in the directions of Z- and Y-axes being controlled by the NC 21.
A numerical control apparatus conventionally used to control the operation of the above-described composite lathe is provided with a CRT display 14a capable of displaying the machining path of the cutting tool 9 thereon as shown in FIG. 3. However, the machining path of a tool such as a drill used in milling whose movement is accompanied by the corresponding motion of the C-axis, is not displayed on the CRT display 14a. This results in a difficulty for the operator to check the progress of milling the workpiece as to the type of hole being drilled (i.e., whether it is a through-hole or a blind hole) or the depth of groove being formed.